A magnetic random access memory (MRAM) is a non-volatile memory which basically includes a magnetoresistive (MR) material, a sense line, and a word line. The MRAM employs the MR effect to store memory states. Magnetic vectors in one or all of the layers of MR material are switched very quickly from one direction to an opposite direction when a magnetic field is applied to the MR material over a certain threshold. According to the direction of the magnetic vectors in the MR material, states are stored, for example, one direction can be defined as a logic "0", and another direction can be defined as a logic "1". The MR material maintains these states even without a magnetic field being applied. The states stored in the MR material can be read by passing a sense current through the cell in a sense line because of the difference between the magnetic resistances of the two states.
In very high density arrays of magnetic memory cells the size of individual cells becomes very small if the array is constructed small enough to be useful in present day electronic devices. As the size of individual cells becomes smaller the aspect ratio (length/width ratio) generally becomes smaller. In two layer magnetic memory cells, e.g. standard tunneling junction cells, as the aspect ratio goes below 5, magnetic vectors in the cells are antiparallel in non-energized (zero magnetic field) conditions. In a copending application entitled "Multi-Layer Magnetic Memory Cells with Improved Switching Characteristics", Ser. No. 08/723,159, filed on Sep. 25 1996, and assigned to the same assignee, methods of reading cells with antiparallel magnetic vectors are disclosed. Also, in a copending application entitled "Magnetic Device Having Multi-Layer s with Insulating and Conductive Layers", Ser. No. 08/834,968, filed on Apr. 7, 1997, and assigned to the same assignees, a dummy magnetic layer is added to a two magnetic layer stack and coupled to one of the two magnetic layers so that the other magnetic layer is a free layer. A drawback of the dummy magnetic layer approach is that it relies on cancellation of magnetostatic interaction between the two magnetic layers and this magnetostatic interaction strength depends on the geometry of the cell and the interlayer spacing. These parameters change as the critical dimension shrinks.
Accordingly, it is highly desirable to provide magnetic random access memories and memory cells which are capable of being written (stored states switched) with less magnetic field.
It is another purpose of the present invention to provide a new and improved multi-state, multi-layer magnetic memory cell with an uncoupled magnetic free layer which switches states with a relatively low magnetic field.
It is still another purpose of the present invention to provide a new and improved multi-state, multi-layer magnetic memory cell with an uncoupled magnetic free layer which can be fabricated very small and with an aspect ratio less than 5.
It is a still further purpose of the present invention to provide a new and improved multi-state, multi-layer magnetic memory cell which is simpler to manufacture and to use.
It is also a purpose of the present invention to provide a new and improved multi-state, multi-layer magnetic memory cell which, because of its size, results in a high density array of cells.